docs: def formatall(self, *args, **kargs):
          " Add-on method for fits returned by chained_nonlinear_fit. "
          ans = ''
          for x in self.chained_fits:
              ans += 10 * '=' + ' ' + str(x) + '\n'
              ans += self.chained_fits[x].format(*args, **kargs)
              ans += '\n'
          return ans[:-1]